In order to maintain the safety of a combustion system, a burner with a stable flame is required. Flame stability of a burner is that quality of a burner which enables it to remain lighted over a wide range of firing rate and fuel/oxidant mixture ratios under practical furnace conditions. Flame stability of a burner is a complex phenomenon influenced, inter alia, by the geometry of the burner and the burner block, the flow conditions of fuel and oxidant, and the temperature conditions of the furnace and the burner block. It is generally believed that the recirculation of hot combustion products near the burner face where fuel and oxidant start to mix is beneficial in enhancing the flame stability of a burner. In order to obtain the desired effects, most air burners are designed with a burner block and often with a swirl in the combustion air flow.
A recent significant advance in the burner art is the aspirator burner and process developed by Dr. J. E. Anderson and described and claimed in U.S. Pat. Nos. 4,378,205 and 4,541,796. By means of this aspirator burner and process one can advantageously employ enriched air and even pure oxygen as the oxidant with resulting significantly improved operating efficiencies. This burner is characterized by a large radial distance between the fuel and oxidant injection points and a relatively high velocity for the oxidant. The flame in a burner such as the aforementioned aspirator burner may be stabilized by the introduction of a small amount of oxygen in an annular stream proximate the fuel stream. A very stable flame is obtained with this arrangement for a broad range of firing conditions.
Flame stabilization by means of a small annular oxidant stream proximate the fuel stream is very effective but is costly and complicated. Two passages must be present in the burner to bring the separate oxidant flows to the face of the burner where they can react with the fuel. This increases the size of the burner and therefore its manufacturing costs. There is also required two separate oxidant supplies, one for the main oxidant and one for the stabilizing annular oxidant. This entails additional piping to the burner, additional valving to control the two oxidant flows, and increased piping and wiring costs to install these additional components. In addition, an annular oxygen passage hinders the cooling of the fuel tube by the water-cooled burner head causing excessive temperatures in this area.
It is therefore an object of this invention to provide a post-mixed burner apparatus and process having fuel and oxidant injection points spaced radially apart which operates with a stable flame without the need for an annular oxidant stream proximate the fuel stream.